Relationship between pressure and surface area of the object can be illustrated using the following activity.
A soft bulletin board, chart paper and drawing pins.
1. Pin the chart paper on a soft bulletin board with flat rounded end of the drawing pin.
2. Now try pinning the chart papers with the sharp pointed ends of the drawing pin.
In the first case, when the rounded end of the pin was used, only an impression of the pin formed on the chart paper.
In the second case, the sharp end makes a hole in the chart paper and hence can be fixed on the bulletin board.
The surface area of the rounded end of the drawing pin is larger than the surface area of the sharp end. Therefore, a greater pressure is produced with the sharp end of the drawing pin.
This proves that, Pressure = Force per unit Area.